The present invention relates to a surface acoustic wave filter and, more particularly, to a surface acoustic wave filter having a flattened passband characteristic of a cascaded double mode SAW filter in which input and output impedances are different from each other.
In recent years, a surface acoustic wave filter is widely used in the field of communications. Since the surface acoustic wave filter has excellent characteristics such as high performance, small size, and manufacturability, it is often used particularly in a portable telephone and the like.
FIG. 5 is a plan view showing the configuration of a conventional filter in which longitudinally coupled double mode SAW filters using the first and the third modes (hereinafter, referred to double mode SAW filters) are cascaded. A first double mode SAW filter is constructed by disposing three IDT electrodes 11, 12, and 13 side by side between grating reflectors 14a, 14b (hereinafter, referred to reflectors) along a direction of propagation of a surface wave on the principal plane of a piezoelectric substrate P. Further, in parallel with the first double mode SAW filter, a second double mode SAW filter is constructed by disposing three IDT electrodes 15, 16,.and 17 side by side between reflectors 18a and 18b. By cascading the first and second double mode SAW filters, a cascaded double mode SAW filter is constructed.
Each of the IDT electrodes 11, 12, and 13 constructing the first double mode SAW filter is made by a pair of interdigital electrodes having a plurality of electrode fingers which positioned between the electrode fingers of the other interdigital electrode. One of the interdigital electrodes of the center IDT electrode 11 is connected to an input terminal IN, and the other interdigital electrode is grounded. Further, one of the interdigital electrodes of each of the IDT electrodes 12 and 13 is connected to an input of the second double mode SAW filter and the other interdigital electrode is grounded. The first and second double mode SAW filters are of symmetrical configuration.
The second double mode SAW filter is constructed almost similar to the first double mode SAW except to the point that the number of pairs of electrode fingers of the center IDT electrode 15 is smaller than that of the IDT electrode 11 in the first double mode SAW filter, and interdigital dummy electrodes D1 and D2 having electrode fingers of the number equal to the balance of the number of electrode fingers are provided and connected to the interdigital electrode on the earth side. By the configuration, an input impedance is set to be low (for example, 50xcexa9) and an output impedance is set to be high (for example, 200xcexa9), thereby realizing impedance matching with a circuit on the ante stage and a circuit on the post stage.
The electrode finger pitches (hereinafter, referred to a pitch) of the IDT electrodes 11, 12, 13, 15, 16, and 17, and the dummy electrodes D1 and D2 are the same.
As it is well known by the person skilled in the art, the first double mode SAW filter shown in FIG. 5 operates as follows. A plurality of surface waves excited by the IDT electrodes 11, 12, and 13 are trapped between the reflectors 14a and 14b and are acoustically coupled. Two longitudinal resonance modes in the first and the third-order are forcedly excited by the IDT electrode pattern. Consequently, by carrying out proper termination, the first double mode SAW filter operates as a double mode SAW filter using the two modes. The pass. band width of the double mode SAW filter is determined by a frequency difference between the first-order resonance mode and the third-order resonance mode. The operation of the second double mode SAW filter is similar to that of the first double mode SAW filter.
The filter is configured as a cascaded type filter so that, attenuation of the filter is set to be higher than that of a single filter and an attenuation of the outside of the passband is increased.
FIG. 6(a) shows a filter characteristic in the case where a 39xc2x0 Y-cut X-propagating LiTaO3 is used for a piezoelectric substrate, 19.5 pairs of electrode fingers are used in the center IDT electrode 11, 13.5 pairs of electrode fingers are used in the center IDT electrode 15, 13.5 pairs of electrode fingers are used in each of the IDT electrodes 12, 13, 16, and 17, the number of each of the dummy electrodes D1 and D2 is six, the number of each of the reflectors 14a, 14b, 18a, and 18b is 200, and the electrode film thickness is 6.5%xcex (xcex is a wavelength defined by the electrode) to thereby manufacture a cascaded double mode SAW filter having a center frequency of 881.5 MHz, a pass band width of 25 MHz, an input impedance of 50xcexa9, and an output impedance of 200xcexa9.
In the conventional cascaded filter in which the input and output impedances are different from each other, however, like the passband characteristic of the filter shown in FIG. 6(a), a large ripple occurs on the low passband side. There is a problem such that the filter is out of the specification of 3.5 dB or less required as the. specification of an RF filter of a portable telephone in the pass band from 869 MHz to 894 MHz (the pass band width is shown by hatched area). Further, as FIG. 6(b) shows a voltage standing wave ratio (VSWR) measured from both the input side (solid line) and the output side (broken line), there is also a problem such that the filter is out of the specification that the hatched VSWR is two or less in the pass band width.
FIGS. 7(a) and 7(b) are Smith charts obtained by measurement from both input and output sides of the cascaded filter, and large cycles are drawn around the passband. As obviously understood from the curve, each of the input and output impedances of the filter is not equal to 50xcexa9. Since the input and output impedances of the filter are designed as 50xcexa9 and 200xcexa9, respectively, an impedance transformer is used to measure the VSWR on the output side.
As means for realizing a flat passband of an RF filter, it is known that an inductance is connected in parallel with a terminal impedance to achieve impedance matching. FIGS. 8(a) and 8(b) are Smith charts measured in such a manner that 50xcexa9 is unchanged on the input side but an inductance of 39 nH is connected in parallel on the output side. FIG. 8(a) is measured from the input side and FIG. 8(b) is measured from the output side via a transformer. It is understood that even when the inductance is connected in parallel on the output side, the impedance characteristic on the input/output sides is not improved.
The present invention has been achieved to solve the problems and its object is to provide a cascaded filter having a flat passband characteristic and having the VSWR characteristic within a desired specification (2 or less).
To achieve the object, the invention according to claim 1 relates to a cascaded surface acoustic wave filter in which longitudinally coupled double mode SAW filters using the first and the third order modes are cascaded, in each of the double mode SAW filters, three IDT electrodes are disposed side by side between grating reflectors in a direction of propagation of a surface wave on the principal plane of a piezoelectric substrate, characterized in that a pitch of the IDT electrode in the center of one of the double mode SAW filters in said filter is set as L2, the number of electrode fingers of the IDT electrode is smaller than that of an IDT electrode disposed in the center of the other double mode SAW filter, and dummy electrodes having a pitch Ld are disposed of the number equal to the difference between the number of electrode fingers of the IDT electrode and that of the IDT electrode in the other double mode SAW filter, and the pitch L2 is set to be higher than an electrode finger pitch L1 of the other IDT electrodes.
The invention according to claim 2 relates to the cascaded surface acoustic wave filter according to claim 1, characterized in that 1.001 less than L2/L1 less than 1.015.
The invention according to claim 3 relates to the cascaded surface acoustic wave filter according to claims 1 or 2, characterized in that the pitch Ld of the dummy electrode is set to be equal to L1.
The invention according to claim 4 relates to the cascaded surface acoustic. wave filter according to claims 1 or 2, characterized in that the pitch Ld of the dummy electrode is set to be equal to L2.